I've read dozens of times (here and elsewhere) that burrs must be sharp. The first piece of advice when buying a used electric grinder is, "replace the burrs". I did that myself, though now I suspect it wasn't worth the time or trouble.

I've now spent some time (and may do a detailed post) examining, measuring and testing the burrs of the following hand grinders:

Zass (2)
Spong (5)
Pe De (1)
Arcade (several)

And, while the Zass has reasonably sharp burrs, the latter three are as sharp as butter knives. The Spongs (yes I bought several) all can produce dust, if that's what one wants. The Arcade won't because the burrs can't be set close enough, but if they could, it would too, produce dust. The Zasses are actually kind of junky, (not as well made as their reputation among their owners) but they also produce dust.

I'm beginning to think that the physics of high speed grinders must be substantially different than low speed (hand grinders). After all, dull stone burrs have been used to make flour (dust) and macha (dust) for centuries and they turn slowly.

It seems that perhaps the exit holes created by the burrs are the factor that controls particle size, not the sharpness or dullness of the burrs.

When looking at high speed grinders, it seems clear that they are going through an initial stage of breaking down the beans so they will move down to the finer levels and at high speed that will result in -- fracturing the beans AKA "popcorning". A hand grinder is similar, but at low speeds it doesn't act like a $10 whirly blade grinder. It acts more as a crusher than an impact related mechanism. More like a mortar and pestle.

Anyhow, in a week or two I may post some detailed results and photos of actual Peppina shots. I've become pretty good at predicting grind quality using a thumbprint test. I press the grinds lightly with my thumb and if they show the whirls of my thumb, it is a bit too fine for espresso. If the whirls are barely visible, it is just about right.

Bottom line: I think that the conventional wisdom of the need for sharp burrs needs to be revisited. Maybe high speed grinders need them for some reason, which I don't understand, but clearly hand grinders don't.

By the way, the Arcade grinders are great for drip and also make good cupping grinders. Takes only a few seconds, they don't trap much inside, have no chutes (or hoppers the way I use and buy them).

dan

You are looking for the wrong sign of burr wear.

Burrs neither crush nor cut beans. A burr grinder works by forcing the beans into an ever tighter space, whether downwards on a conical grinder, or outwards on a flat burr grinder. The burr themselves criss-cross, and as the grind stone turns the advance of the criss-cross pattern forces the beans into the tighter space. In that tighter space, they crush either against the grindstone or against each other*

The sharpness of the burr's edge is irrelevant to this action. Instead, as the burr wears down, getting less raised from the grindstone's base, it doesn't criss cross as well with its opposite numbers on the other grindstone. The bean slips, and it takes longer to move through the burrs. The slippage probably also changes the ratio of coarse to fine particles along with grinder efficiency.

Imagine that as burrs wore down, their edges stayed razor sharp. This would make no difference to the drop in grind efficiency, their criss-cross action, once they are less raised, would still be compromisd, and the grinder would still lose traction. If on the other hand, the burrs stayed as raised as new, and merely got dull, they would continue grinding just fine.

A ceramic grind stone, like the little Vario's, wears more slowly. So the burrs on it can be much less raised and still have longer life.

*It's a well known fact that the the grind setting needs to be finer when grinding few beans, either when single dosing or when the hopper is near empty. This occurs because with fewer beans in the burrs at any one time, there is less bean to bean crushing action. To get the same fineness, the grindstones have to pick up the slack; and they do this by being closer together.

So, do hand grinders and electric grinders grind the same or do any different physics apply?

The bean doesn't know what is turning the burr; and lots have people have attached drills to Zass grinders for faster grinding. So my guess is that there is no difference in principle. Clearly, the entire assembly holding the burrs has to be more robust for a motorized grinder.

But the difference you noticed is probably real, not for the sharpness of the burrs, but for how big they are. The burr on a motorized grinder has to push the beans through the grindstones more quickly, so presumably (help from scientists here), it has to exert more force on each bean. This means the burr may have to be larger in size where it contacts the bean to get sufficient purchase on the bean's body as it pushes it through.

Here's what I was wondering, Jim:

The slowest Titans go at around 500 rpm, the faster ones at three times that. So a bean inside of that spinning burr is going to bounce around a lot and centrifugal forces will definitely come into play. A hand grinder it is going at 40-80 rpm (max) and I don't think those forces will be there. On a vertical hand grinder, there is no significant centrifugal force that will fling the beans around. That's why the hand grinders usually have more of an auger to introduce the beans into the smaller burrs.

I think that the power grinders, with one exception, need the centrifugal forces to force the powder out the sides. So they are relying on the speed. Hand grinders are quite different in that respect. To grind 14 gms on a power grinder the burrs are probably spinning several hundred times at least. On a hand grinder they spin 60 -120 times. There is something different between the two, perhaps.

Another question, if you don't mind: does 'popcorning' really result in a taste difference?

Thanks,

dan

I'll defer to any real scientists that show up, but a faster rotating burr shouldn't need to exert more force on each bean to fracture it. But it will increase capacity and likely heat dissipation as well due to mechanical inefficiencies.

In an ideal system at least, the amount of energy to fracture a coffee bean will be constant. Once the applied mechanical stress (e.g. impact, compression, shearing force) exceeds the yield value of the bean, it will deform and then fracture. This can be represented in the general form dE/dD = cDexp-ne , where E is the energy to break a mass unit of diameter D and c and ne are constants. This is variously referred to as the Rittinger, Kick and Bond's laws to estimate energy requirements for different extents of size reduction. Of course a coffee grinder is not an ideal, perfectly efficient system so changing the burr speed will impact grinding efficiency.

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wookie wrote: ... a faster rotating burr shouldn't need to exert more force on each bean to fracture it..

I didn't think of that. I was thinking it required more force to push the bean into a tighter space. But, in effect, pushing the bean into a tighter space means breaking it into pieces, so pushing it faster means breaking it more frequently. The force for each break is the same, but the frequency at which that force is applied is greater the faster the grinder rotates. At this point, my intuition takes a vacation, and I don't have a feel why exactly this would require beefier burrs.

DJR wrote: The slowest Titans go at around 500 rpm, the faster ones at three times that. So a bean inside of that spinning burr is going to bounce around a lot and centrifugal forces will definitely come into play. A hand grinder it is going at 40-80 rpm (max) and I don't think those forces will be there. ...
Another question, if you don't mind: does 'popcorning' really result in a taste difference?

I don't think beans bounce around once they are inside the grindstones, the burrs are pushing them against the contracting walls of the grindstones and keep them snug. Popcorning is this: In the absence of other beans or an augur to to push them in; the beans bounce around at the top of the grindstones.

This is why popcorning is unlikely to change the taste, (at least according to my blind tests). Popcorning doesn't happen as the beans are ground, but before beans enters the burrs. Granted, low dose grinding requires setting the grindstones closer together, since the slowed down feed rate means the beans are not as frequently broken by other beans, and need to be broken by the stones instead. But unless one can prove that being broken against another bean is different from being broken against a piece of metal, there is no physical reason to believe the taste will change.

As far as my taste buds go, there's no change of taste. Also, there's no change of quality of the flow. Only the grind setting needs to be adjusted finer for single dose.
I appreciate your explanation, Jim, as to why the grind setting occurs. Me was thinking that due to lack of other beans pushing the bean into the burrs, it gets broken by the grindstones while jumping... back and forth and that this is popcorning. But, probably, my imagination was quite not exactly right.

Apropos to this discussion, my Robur is equipped with a variable frequency drive, allowing the burr speed to be easily varied between 500 rpm (at 60 Hz) and 300 rpm (at 36 Hz). I like to use 36 Hz because the machine runs with a refined, civilized whirr instead of the menacing howl it makes at 60 Hz. Two interesting data points:

1. The rate of grind varies very little with burr speed (~5g/sec regardless).
2. Increasing the burr speed results in a coarser grind. An increase in frequency of 10 Hz is about equal to adjusting the grind coarser by about 1.5 fine lines on the collar.

I haven't noticed much flavor difference when comparing equivalent shots ground at various speeds. But if anyone wants to come by with 3-5 lbs of their favorite coffee, we could do some blind (and deaf) testing; maybe we could identify some trends.

AndyS wrote: 2. Increasing the burr speed results in a coarser grind. An increase in frequency of 10 Hz is about equal to adjusting the grind coarser by about 1.5 fine lines on the collar.

I used to think that the grind fineness was entirely determined by how close the grindstones were together at the exit point. But that didn't explain why single dose grinding was coarser than full hopper grinding at the same grind setting. Your observation is in line with the idea that grind fineness also depends on how often a bean gets broken on its way through the grindstones. It seems that when it goes through faster, it does so with fewer breaks.

The spacing of the grindstones sets the maximum grind size, but the particles can be smaller. Presumably when you grind slowly or with a full hopper, the average size is a good deal smaller than this maximum size, while it's closer for single dose or fast grinders. This either means a wider dispersion of coarse particles or more fines when grinding with a full hopper or more slowly.